Process of recovering zinc.



F. IL/HST.

PROCESS or RECovEmNG ZINC.

APPLICATION FILED JAN. 29. l9l8.

Patented Oct. 8, 1918.

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FREDERICK LAIST, OF ANACONDA, MONTANA, ASSIGNOR T ANACONDA COPPER MINING COMPANY, OF ANACONDA, MONTANA, A CORPORATION OF MONTANA.

PROCESS OF RECOVERING ZINC.

Specification of Letters Patent.

Patented Oct. 8, 1918'.l

Application led January 29d, 1918. Serial No. 214,310.

To all whom z't may concern:

Be it known that I, FREDERICK LAIs'r, a citizen of the United States, residing at Anaconda, in the county .of Deerlodge and State of Montana, have invented certain new and useful Improvements in Processes of Recovering Zinc, of which the following is a specification. y

It is well known that in the hydrometallurgical treatment of silicius zinc ores the presence of gelatinous silica at certain stages of the process interferes seriously with the operation, -notonly by clogging the filters,

but 'also by persistently retaining considerable volumes of rich zinc-bearing solution,

and thereby reducing the recovery of zinc.

The present invention is a process whereby these difficulties are largely overcome and materially increased yields of zinc are obtained. A preferred embodiment of. the

process is as follows:

Concentrates containing for example about 33 per cent. zinc, eight per cent. lead, 20 per cent. iron and six per cent. insoluble matter,

the last comprisn about 4.5 per cent. silica and 1.5 per cent. a umina, are roasted in lmultiple hearth furnaces in order to oxidize the sulids to oxidsland sulfates. The roasted product, or calcine, is then leached with a solution containing about eight to nine per cent. of sulfuric acid, this solution in 'a repetition of the process being the depleted electrolyte from the cells inl which the zinc is electrolytically precipitated. Usually apas proximately five tons of leaching solution are used per ton of calcine. The resulting solution is then neutralized or nearly neutralized with calcine, and thereafter a small proportion of limestone is usually added. The

effect of this limestone addition is to establi s h a slight alkaline condition in the liquor, and therebyl to precipitate in gelatinous form any silicawhich has been dissolved during the acid treatment. Practically, it

visfnecessary to effect complete precipitation o the silica at this point, since 1t will otherwise separate during later operating stages and occasion a great deal of trouble.

The precipitation of silica may be rendered even more complete by adding to the pulp, after the limestone addition2 a small amount of zinc du'st; this reagent, elther in presence or absence ofcopper salts, is very effective for precipitatin any residual traces of dissolved silica. n presence of copper salts the cement copper is of course the active reagent. v f

The above-described operations of neutralizing the liquid and precipitating the silica are conveniently performed in Pachuca tanks with thorough stirring. At this point the soluton should carry at least 8O per cent. of the zinc content of the original calcine. The4 Pachuca tanks are now discharged into Dorr'settling tanks or thickeners, andthe 6,5 thickened residue, together with the precipitated matters, passes to the iilters. The -filtrate, together with the overflow from the. Dorr thickeners is a zinc solution usually containing 'small amounts of copper and cadmium. These metals are precipitated by zinc in 'dust or other suitable form, the solution clarified by settling or filtration and l passed to the electrolytic cells) The entering electrolyte is a neutral solution containing about eight per cent. of zinc in the form of sulfate: the depleted electrolyte flowing from the electrolytic cells contains some two per cent. of zinc as sulfate, together with about eight to nine per cent. of sulfuric acid, and ist-returned to the leaching tanks as mentioned above. Leadanodes may be used, andthe current conditionsare preferably those which have become standard in this operation. 85

In the practice of the above process there is obtained a filter cake which comprises the insoluble matter from -the ore and the precipitated gelatinous silica. This is in the form of a wet cake, usually containin 42-45 per cent. of moisture, which is o course in reality a rich zinc-sulfate solution' It is impossible to wash the cake free froni= zinc with an amount of water'within'practicable limits, and the losses at this point 95 may attain 15 per cent. of the zinc contentof the ore.

Efforts have been made to reduce this loss by removing the leach liquor from' the residue before neutralizing and while still 100 containing about 0.5- per cent. of acid. The acid pulp was passed through thickeners and filters, and yielded a dry, sandycake, containing only about 20 er cent. of moisture and readily washed. owever, the sub 105 sequent treatment of the acid solution with calcine, limestone and zinc dust yielded an '.unworkable precipitate of gelatinous silica,

this precipitate being slimy, slow-settling, and producing a cake containing upto S0 per 110 mail cent. of moisture. The necessity of precipitating the gelatinous silica in presence of the sandy ore residue was therefore established.

According to the present invention the mechanical diliiculties due to the precipitationof gelatinous silica, as Well as the eXcessive losses of zinc occasioned thereby, are lvoided by proceeding substantially as folowsz- The silica is precipitated in presence of the ore-pulp, preferably in the manner first described above, that is, by successive additions of calcine, limestone, and zinc dust, if required, the latter either with or without copper salts. The subsequent 'operations of thickening and filtration are easily carried out in apparatus of standard type, but yield as above stated a'filter cake of high moisture-content and rich in zinc.

This filter cake, Which maybe termed the primary fitter cake and which need not be Washed, is discharged from the press and dried or gently' calcined at a temperature sufiiciently high to effect dehydration of the silica, as Well as of the hydroxids of alumina' and iron, a temperature of about 150"y C. being suitable. These compounds are thereby rendered insoluble in Water or dilute acids, and are also converted into a granular condition Which lends itself readily to filtration, the zinc values meantime remaining soluble. The dried residue is now leached with a sufcient quantity of acidified solution, so that, after the leaching is completed, a sm'all excess of acid may remain. An 8% solution of sulfuric acid is suitable for this purpose. rlhe pulp is now settled or thickened, filtered, and Washed. The Washing is readily accomplished in the filter, and the residue, termed the secondary filter cake is sandy or granular. The filtrate from the secondary filters, together with the overflow "from `the secondary settling tank, is acid,

and is returned to the primary leaching tanks, being preferably introduced just bel drawing in which the figure is a flow-sheetl diagrammatically illustrating a preferred embodiment of the complete process.`

I claim 1. In a process of recovering zinc from silicious calcine, the steps which consist in leaching the calcine With an acid liquor,-

neutralizing the liquor and thereby yprecipitating silica in presence of the insoluble residue, separating the zinc-bearing solution from the residue2 dehydrating the residue, and recovering zinc values therefrom by leaching.

2. ln a process of recovering zinc from silicious calcine, the steps' which consist in leaching the calcine with an acid liquor, neutralizing the liquor and thereby precipitating silica in presence of the insoluble residue, separating the zinc-bearing solution from the residue, dehydrating the residue` recovering zinc values therefrom by leaching with an acid solution and returning the resulting zinc-bearing solution to the primary leaching operation.

3. ln a process of recovering zinc fronr silicious calcine, the steps which consist in precipitating dissolved silica in gelatinous form from a Zinc-bearing solution, separating and dehydrating the precipitate, and recovering zinc from said dehydrated precipitate by leaching.

lln testimonywhereof l aliix my signature.

FREDERICK LAlS'll. 

